A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
In the manufacture of devices using lithographic processes, each mask pattern is typically projected onto the target portion in focus. In practice, this means that the target portion of the substrate is positioned in a plane of best focus of the aerial image projected by the projection system. As the critical dimension (CD) (i.e., the dimension of a feature or features in which variations will cause undesirable variation in physical properties of the feature, such as the gate width of a transistor) in lithography shrinks, consistency of focus, both across a substrate and between substrates, becomes increasingly important.
The use of an alignment system to monitor focus has been proposed and involves printing focus-sensitive alignment markers at known positions relative to normal alignment markers at various different focus settings, i.e., positions of the substrate relative to the projection system. The position of these focus-sensitive markers with respect to the normal alignment markers is measured and an alignment offset (AO) can be determined which is representative of focus errors.
One way of verifying the quality of focus control in lithographic tools is by using the Leveling Verification Test (LVT). The LVT test uses a special reticle with glued glass wedges on top, to locally create non-telecentric illumination on a double telecentric lens. This non-telecentric illumination is used to cause a lateral shift in x, y as function of defocus z of the aerial image of an XPA alignment mark situated beneath a glass wedge. By measuring the alignment shift of this defocus mark with respect to XPA reference mark (imaged without wedge on top), the defocus at the moment of exposing can be determined.
A main drawback with the LVT test is that the read-out of the wafer is done by the alignment system present on the lithographic tool itself, thus reducing the time available for production.